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Axelrod: With less than 50 days to the Iowa caucus, the GOP race is getting serious

He says key candidates to watch are Trump, Cruz, Rubio and Christie

David Axelrod is CNN's senior political commentator and host of the podcast "The Axe Files." He was senior adviser to President Barack Obama and chief strategist for the 2008 and 2012 Obama campaigns. The opinions expressed in this commentary are his.

(CNN)"Now the real game begins!"

As we look forward to the @CNN debate Tuesday, the immortal words of the Riddler, Batman's colorful nemesis, ring true.

David Axelrod

The first CNN debate in September was a relatively tame affair -- or as tame as a debate featuring Donald J. Trump can be. The candidates were establishing their messages and their places in the race.

Jeb Bush, still thought to be a principal contender, was featured center stage, next to Trump. Ben Carson, on Trump's other side, had not yet begun his rocket ride up -- and down. Ted Cruz and Marco Rubio were on the flanks.

My, how things have changed!

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In the ensuing two months, Trump's support has strengthened if not grown, helped by his limitless gall and by terrorist attacks that have created a greater market for his strongman appeal.

Bush, beset by miscues, the weight of his family legacy and Trump's withering assault on his diffident style, has faded. And Cruz and Rubio -- a pair of freshman senators with similar profiles but distinct approaches -- are on the rise.

With less than 50 days to go before the Iowa caucus, the GOP race is taking shape. The starting line is within sight. And so Tuesday's final GOP debate before the holidays and election year promises to be the most intriguing and contentious yet.

Here are the things I'll be watching for:

How do they navigate Trump?

As The Donald has noted, those who have launched direct attacks on Trump have generally fared badly. Rick Perry was the first to lash Trump as a plague on conservatism. One suspects the former Texas governor's eagerness to engage was prompted by his own failure to connect with voters. If so, the gambit failed badly. He was the first out of the race.

Bush, Carly Fiorina, senators Rand Paul and Lindsey Graham and Gov. John Kasich of Ohio all have taken turns tangling with Trump, fetching gleefully brutal rejoinders from a frontrunner with an uncanny instinct for the jugular.

Trump also has cast all his attackers as exemplars of a Republican establishment that is reviled by the activist wing of the GOP.

Perhaps not entirely by coincidence, the three contenders who have shown some positive movement in recent polls have been those who have tried to steer clear of Trump -- Cruz, Rubio and Gov. Chris Christie of New Jersey.

Cruz, who has plotted his race brilliantly, has made quantum leaps, seizing a 10-point lead over Trump in the latest Iowa Poll. He has done it by methodically organizing among the social conservatives and tea party and, in his own words, killing Trump and Carson with kindness, waiting for their collapse to pick up the pieces.

Carson appears to have cooperated. His precipitous drop in Iowa and nationally after stumbling on national security issues clearly has redounded to Cruz's benefit.

But Trump remains a stubborn obstacle to Cruz's plan to coalesce the anti-government populists.

Businessman Donald Trump announced June 16 at his Trump Tower in New York City that he is seeking the Republican presidential nomination. This ends more than two decades of flirting with the idea of running for the White House.

"So, ladies and gentlemen, I am officially running for president of the United States, and we are going to make our country great again," Trump told the crowd at his announcement.

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Photos:Who's running for president?

Sen. Ted Cruz of Texas has made a name for himself in the Senate, solidifying his brand as a conservative firebrand willing to take on the GOP's establishment. He announced he was seeking the Republican presidential nomination in a speech on March 23.

"These are all of our stories," Cruz told the audience at Liberty University in Virginia. "These are who we are as Americans. And yet for so many Americans, the promise of America seems more and more distant."

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Photos:Who's running for president?

Ohio Gov. John Kasich joined the Republican field July 21 as he formally announced his White House bid.

"I am here to ask you for your prayers, for your support ... because I have decided to run for president of the United States," Kasich told his kickoff rally at the Ohio State University.

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Photos:Who's running for president?

Hillary Clinton launched her presidential bid on April 12 through a video message on social media. The former first lady, senator and secretary of state is considered the front-runner among possible Democratic candidates.

"Everyday Americans need a champion, and I want to be that champion -- so you can do more than just get by -- you can get ahead. And stay ahead," she said in her announcement video. "Because when families are strong, America is strong. So I'm hitting the road to earn your vote, because it's your time. And I hope you'll join me on this journey."

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Photos:Who's running for president?

Sen. Bernie Sanders, an independent from Vermont who caucuses with Democrats, announced his run in an email to supporters on April 30. He has said the United States needs a "political revolution" of working-class Americans to take back control of the government from billionaires.

"This great nation and its government belong to all of the people and not to a handful of billionaires, their super PACs and their lobbyists," Sanders said at a rally in Vermont on May 26.

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Is the Trump-Cruz bromance over?

With Cruz now nipping at his heels -- and dissing The Donald in private fundraising meetings, a recording of which surfaced last week -- will Trump aim his missiles at Cruz ‪on Tuesday night? And if he does, will Cruz quit the bromance and fire back?

Cruz and Rubio also have been spatting, as the reality crystallizes that they may be headed for a showdown.

If Cruz hopes to emerge as the candidate of the aggregated forces of the right, Rubio has emerged as a potential life raft for the GOP's establishment forces, who have seen the Bush battleship and Kasich schooner take on water.

To date, Rubio has played the game more artfully than any, maintaining his standing with all factions of the GOP -- the establishment, social conservatives and tea party.

Will Cruz or Trump try and split the anti-establishment forces try and prosecute Rubio for his abortive collaboration with a bipartisan group of senators on immigration reform?

Will Bush or Kasich, desperate to get back into the game, shoot at Rubio for being scared off of his immigration positions?

Photos:The many facial expressions of Donald Trump

Many people on the Internet decided that GOP candidate Donald Trump is the most expressive person running for president. Here's a look at his many facial expressions:

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Photos:The many facial expressions of Donald Trump

Trump speaks about illegal immigration July 10 at the Beverly Wilshire Hotel in Beverly Hills, California.

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Photos:The many facial expressions of Donald Trump

Trump speaks to guests gathered for a campaign event at the Grand River Center in Dubuque, Iowa, on August 25.

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Photos:The many facial expressions of Donald Trump

Trump visits his Scottish golf course Turnberry on July 30.

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Photos:The many facial expressions of Donald Trump

Trump speaks during a rally August 21 at Ladd-Peebles Stadium in Mobile, Alabama.

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Photos:The many facial expressions of Donald Trump

Trump eats a pork chop on a stick while attending the Iowa State Fair on August 15.

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Photos:The many facial expressions of Donald Trump

Trump flashes a thumbs-up as he arrives for the start of the first presidential debate August 6 in Cleveland.

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Photos:The many facial expressions of Donald Trump

Trump participates in the Republican debate in Cleveland.

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Photos:The many facial expressions of Donald Trump

Trump arrives for jury duty in New York on August 17.

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Photos:The many facial expressions of Donald Trump

Trump walks his Turnberry golf course on July 30.

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Photos:The many facial expressions of Donald Trump

Trump at Turnberry on July 30.

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Photos:The many facial expressions of Donald Trump

Trump talks to the media in Laredo, Texas, during a trip to the Mexico border on July 23.

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Photos:The many facial expressions of Donald Trump

Trump greets onlookers after taping an interview with Anderson Cooper at a Trump-owned building in New York on July 22.

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Photos:The many facial expressions of Donald Trump

Trump exits New York Supreme Court after jury duty on August 17.

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Terrorism a top concern

Another change since September: The terrorist attacks in Paris and San Bernardino have profoundly changed the backdrop for this debate. Terrorism and national security have jumped past the economy to the top of the list of voter concerns, at least for now.

Trump's bellicose response -- first to bar Syrian refugees and then all Muslims -- created a furor but did nothing to diminish his support. He's sure to be questioned closely on ‪this Tuesday night, and may welcome it. Polls shows that while a majority of Americans oppose his stances, a majority of Republicans support them.

Rubio, a self-styled national security hawk, has targeted Cruz, after the Paris attack, for supporting changes in the collection of phone and Internet data that Rubio says imperil the country.

For Rubio, the national security attack is a way of getting to Cruz's right without jeopardizing his own standing with establishment Republicans.

Expect to hear more of this on Tuesday.

The Christie factor

While Christie continues to lag in national polls, he has encamped in New Hampshire, received the blessing of the Manchester Union-Leader and has risen to second place behind Trump in the state that will host the first-in-the-nation primary.

Christie, a former prosecutor, has, as much as any of the candidates, seized on terrorism, vowing muscular leadership.

A strong showing for Christie in New Hampshire may not propel him into the upper echelons of the race, given his lingering baggage of New Jersey scandals and fiscal woes. But it could spell doom for establishment candidates like Bush and Kasich and make Rubio's path more difficult.